Are all palaces temporary, or can some hold information permanently?

Both — "scratch" palaces hold throwaway lists briefly, while dedicated palaces, kept under review, can store knowledge for years.

The difference isn't the palace, it's the review schedule. Stop reviewing and the images fade (fine for a shopping list). Keep reviewing at expanding intervals and the same palace becomes a durable store — competitors and lifelong learners keep permanent palaces for poems, decks of cards, or subject knowledge. Decide up front whether a palace is disposable or permanent, and review (or don't) accordingly.